Introduction
The timing of surgical intervention for patients with mitral or aortic regurgitation often depends on the accurate assessment of the severity of the valvular insufficiency and its effect on left ventricular (LV) size [
1,
2]. Two-dimensional transthoracic and transesophageal echocardiography are most commonly used to assess mitral and aortic regurgitation. Published guidelines recommend the use of LV ejection fraction as well as linear measurements such as LV end-diastolic dimension (EDD) and LV end-systolic dimension (ESD) to determine the timing of surgical interventions [
2]. Cardiovascular magnetic resonance (CMR) is an accurate method for quantifying the severity of mitral and aortic regurgitation [
3‐
7]. CMR is more accurate and reproducible than two-dimensional echocardiography in the three-dimensional volumetric evaluation of LV size and function [
8‐
14]. An important limitation of the current guidelines is that no volumetric parameters are provided for determining the timing of surgery and we currently rely on linear measurements as a surrogate for describing LV volumetric changes. The purpose of this study is to characterize the physiological relationship between regurgitant volume and LV volume with CMR in patients with mitral and aortic regurgitation.
Discussion
CMR has long been accepted as an accurate and reproducible technique for evaluating LV size and function. Good correlation has been obtained by comparing CMR to other established techniques such as echocardiography [
11,
16], contrast ventriculography [
17], and radionuclide imaging [
18,
19]. In addition, several groups of investigators have documented the accuracy of CMR for quantifying regurgitant volumes in patients with AR or MR, again by comparison to other imaging modalities such as echocardiography [
6,
20‐
22] or cardiac catheterization [
5]. A problem with these comparative studies is that it can be difficult to determine which of the two imaging modalities is correct when the results differ, which they inevitably do to at least some extent.
To our knowledge, this is the first CMR study to quantify the physiologic relationship between regurgitant volume and chamber volume in patients with isolated chronic AR or MR and preserved LV function. For both AR and MR, we find a strong linear correlation between regurgitant volume and LV EDVI (r2 = 0.8). This provides an independent line of evidence supporting the accuracy of MRI for quantifying regurgitant volume and ventricular volumes. We also find MRI is a robust technique. The data show excellent interobserver variation for quantifying regurgitant volume as demonstrated by the Bland-Altman analysis which showed a mean difference of 0.6 ± 4 ml for AR and 4 ± 6 ml for MR.
In our patients with chronic mitral regurgitation, we find the correlation between regurgitant volume and LV EDVI is stronger than with LV ESVI, probably in part because additional factors influence LV ESVI, such as myocardial contractility. The coupling between LV EDVI and regurgitant volume is strong. An r
2 of 0.8 implies 80% of the observed variation in LV volume is due to the severity of the regurgitant volume. This implies that in our patient population only 20% of the variation is related to other factors such as differences in pre-load, afterload or heart rate. It is possible that an acute physiologic change could weaken the coupling between regurgitant volume and LV EDVI, because regurgitant volume is an instaneous measure whereas LV EDVI is a more chronic measure, and may better reflect the long term average severity of regurgitation. It is interesting to note that LV ESD, a parameter included in the AHA/ACC management guidelines to guide therapy, correlates very poorly with regurgitant volume (r
2 = 0.2). Finally, left atrial volume, a parameter which is often used by the echocardiographic community and which has been reported as a reliable estimator of regurgitant volume [
23], shows a relatively poor correlation with regurgitant volume (r
2 = 0.3).
In our patients with chronic AR, we find a stronger positive correlation with LV EDVI than with LV ESVI, again probably in part because additional factors influence LV ESVI, such as myocardial contractility. It is interesting to note that for a given regurgitant volume, patients with AR have more LV enlargement and a greater LV mass index than patients with MR. This observation is likely due to the nature of the hemodynamic stress placed on the LV; whereas MR is a pure volume lesion AR is both a pressure and a volume lesion [
24]. Additionally, our data suggest that the difference between the two regression lines reflects the difference in afterload of the two patient groups. As one expects, the two linear regression lines approach the same value at a regurgitant volume of 0. Commonly used linear measures such as LV EDD and LV ESD showed a substantially worse correlation with regurgitant volume than volumetric measures such as LV EDVI or LV ESVI. These finding are consistent with previous reports that a linear measurement does not always accurately represent the actual volumetric changes of a 3-dimensional structure such as the LV [
25].
Many of the patients in this study were referred for CMR because the severity of the regurgitation could not be accurately determined by echocardiography. The clinical CMR reports included the regurgitant volume, LV dimensions, and LV volumes. Unfortunately, the LV volumes could not be used by the referring physician to determine the timing of surgical intervention because the AHA/ACC management guidelines only refer to linear dimensions. However, the fact that the volumetric measures correlated better to the regurgitant volume than corresponding linear measures suggests the possibility that volumetric measures may be more valuable than linear measures in assessing the response of the left ventricle to chronic MR or AR and in guiding the timing of surgical intervention. It is our hope that the volumetric correlates to the guideline dimensions which are listed in Table
2 will provide a foundation for the inclusion of volumetric data in future management guidelines.
Competing interests
The authors have no competing interests that relate to this study. Dr Steven Wolff is the owner of NeoSoft, LLC and NeoCoil, LLC.
Authors' contributions
SU and SDW designed the study, performed data analysis, and prepared the manuscript. AS performed statistical analysis. OS and CC performed image acquisition. SK, PN performed data collection. FC performed data analysis. All authors read and approved the final manuscript.